1. Field of the Invention
This invention relates to heating elements comprising conductive polymers.
2. Description of the Prior Art
It is well known that polymers can be made electrically conductive by dispersing therein suitable amounts of finely divided conductive fillers. It is also well known that certain polymeric articles can be rendered heat-recoverable. It has been proposed (see British Pat. No. 1,265,194) to make a heat-recoverable article comprising a first heat-recoverable member composed of a conductive polymer and a second heat-recoverable member which is not electrically conductive, and to cause such an article to recover by passing an electric current through the first member. However, such articles suffer from certain disadvantages. In particular the electrical characteristics of the conductive polymer member are liable to change excessively if any of the dimensions of the member are changed by more than 30%, which is less than is generally desirable for heat-recoverable articles. Furthermore, the presence of the conductive polymer layer increases the force needed to deform the article and can adversely affect recovery. It has also been found that if the current is passed from end to end of the conductive polymer member, as suggested by British Pat. No. 1,265,194, the member is often not heated as uniformly as is desirable to achieve satisfactory shrinkage and to avoid local overheating.